One of the reasons I bought the Denon 3808CI was for its network capability via Ethernet. However, Denon's own internal media server ("Denon Network Audio") very rarely finds my Vista PC. Others have had similar issues. Things to try include:

Ensure that Windows Media Player is running before booting the receiver

Ensure that the Windows service, "Windows Media Player Network Sharing Service" is ON

Denon support recommends turning OFF UPnP on your router

Check that your firewall is not blocking something needed

Try restarting Windows Media Player and the Nework Sharing Service

However, after fussing around with the above and other tests, I still find that the Denon just very rarely locates my PC. However, it plays Internet radio, so it's got an OK connection to the Internet; it's just not registering my PC.